This item appears to be a common cloth sack about 4 feet by 6 feet in size. However, it opens in a nondimensional space: it's bigger on the inside. Treat it as a Type III Bag of Holding with ten times the listed maximum weight and volume for all purposes except as listed here.
The bag has one important restriction: all non-bony matter is immediately expelled from it the moment it is no longer held or carried by a creature.
Creatures can attempt to enter the bag, but they must make a DC 20 constitution check or have their skeletons (if they possess any) ripped out and pulled into the bag while remaining outside themselves. This sets their strength and dexterity scores to 0 and deals 20d6 points of damage. The damage can be healed normally: the lowered ability scores are only returned to their previous value if the creature's complete skeleton regrows, whether by natural regeneration, a Regenerate spell, or any other effect able to do so. A creature without bones is incapable of entering the bag. A creature that consists solely of bones (such as a skeleton) can enter the bag
Moderate Conjuration; CL 11th; Craft Wondrous Item, Black Bag, Fleshshiver
Bag of Flameskulls of Holding (Cursed)
This item functions as a Bag of Skulls of Holding and appears to be one when subjected to most forms of magical identification. However, it has one major difference: if a skull that used to belong to a spellcaster is placed within the bag, it transforms into a Flameskull (Lost Empires of Faerun). The next time the bag is opened, the Flameskull bursts forth and attacks the creature to open it to the best of its ability.
Multiple flameskulls can occupy the bag at the same time.
Moderate Conjuration; CL 11th; Craft Wondrous Item, Black Bag, Create Undead
Bag of the Skull Harvester
This item functions as a Bag of Skulls of Holding and appears to be one when subjected to most forms of magical divination. However, it has one major difference.
If no skulls have been placed in the bag for 24 hours, the next intelligent creature that touches it must make a DC 25 Will saving throw or be compelled to kill the nearest creature and place its skull in the bag as soon as possible. After completing this task, the creature loses all memory of its atrocity.
Moderate Conjuration; CL 11th; Craft Wondrous Item, Black Bag, Modify Memory.